The controlled electrolytic generation of gases is useful to convert chemical to mechanical energy in a variety of applications. For example, a variety of lubricant or fluid delivery systems driven by the electrolytic generation of a gas are known. For example, U.S. Pat. No. 4,023,648 to Orlitzky et al. (1977) shows a lubricant applicator driven by gas generated in an electrochemical cell and provides a method for the electrochemical generation of hydrogen gas.
Fluid dispensers driven by electrochemically generated gases, and other electrochemical transducers may often be used in circumstances which give rise to special operational requirements. Typically, components of any electrolytic cell used in such an application must be stable over time and over a range of temperatures. In such devices, it may be undesirable to have highly reactive gases generated, such as hydrogen or oxygen. Once the circuits are closed to initiate electrolytic gas generation, it is generally desirable to have relatively fast electrode reactions with low overpotential (i.e. a small difference between the electrode potential under electrolysis conditions and the thermodynamic value of the electrode potential in the absence of electrolysis), small concentration polarisation of solutes across the cell (i.e. rapid diffusion of reactants to the electrode surfaces), and small separator resistance effects (i.e. little resistance caused by solid separators within the cell. It is also desirable to produce gases from a small amount of material, i.e. to have efficient gas generation and high stoichiometric coefficients for gaseous reaction products.
The electrochemical generation of a gas can be represented by equation (1):aR+/−ne−→bG+cPwhere R, G and P represent the reactant, the gas product, and the non-gas product respectively; and a, b, c, and n are the stoichiometric coefficients. When utilizing an electrical circuit to drive the current through the electrochemical cell it is desirable to produce gas in an efficient manner from a viewpoint of electric charge consumption. Such efficiency requires a high gas product stoichiometric coefficient associated with a low electron stoichiometric coefficient. A stoichiometric efficiency of gas generation (E) in moles per Faraday may be defined in equation (1) as:E=b/n mol/F
Hydrogen and oxygen gases are used in a variety of known electrochemical gas generators. For example the anodic oxidation and cathodic reduction of water respectively generate oxygen and hydrogen by the reactions 1 and 2:Anodic oxidation of H2O: 2H2O−4e−→O2+4H+  reaction1Cathodic reduction of H2O: 2H2O+2e−→H2+2OH−  reaction2
The anodic oxidation of water has a low stoichiometry efficiency for gas production (0.25 mole of oxygen gas per Faraday). A low stoichiometry efficiency may be undesirable because the quantities of reactant and current needed to produced the desired amount of gas may require a large volume of the unit and a high capacity energy source. Another disadvantage of oxygen is that it may pose safety problems when utilized for dispensing combustible fluids such as grease.
The cathodic reduction of water has a better stoichiometric efficiency for gas production (0.50 mole of hydrogen gas per Faraday) but the production of hydrogen gas is hazardous due to its explosive reactivity with oxygen upon ignition. Another disadvantage of hydrogen is that it diffuses relatively rapidly through a variety of polymeric barriers that might otherwise be used to contain the electrolytically generated gas in a mechanical transducer, such as a fluid dispenser.
Nitrogen is a relatively inert gas that may usefully be produced by electrolytic reactions to provide controlled amounts of gas. U.S. Pat. No. 5,567,287 issued to Joshi et al. (1996) discloses a solid state electrochemical nitrogen gas generator for fluid dispensing applications wherein nitrogen is produced by electro-oxidation of alkali metal nitrides or azides. The azide half-cell reaction in that system produces non-reactive nitrogen with a stochiometry efficiency of 1.5 moles of nitrogen gas per Faraday (reaction 3).2N3−→3N2+2e−  reaction 3
Based on reaction 3, a fluid dispenser operating at 0.25 mA has the potential to generate about 0.33 ml STP of gas per hour for up to 4000 hours from a battery with capacity of 1 A.h. With sodium azide as the anode reactant, 1 litre STP of nitrogen gas could be generated from about 2 grams of NaN3.
The azide half-cell reaction in such a system may however be slow, in part because of the high overpotential required for the electro-oxidation of azide. To overcome the problem of the sluggish kinetics of the azide half-cell, additives such as thiocyanate may be used to catalyse iodine mediated formation of nitrogen from azides. However, such systems suffer from the disadvantages that azides are toxic and the thiocyanate salt catalysts are also toxic. The presence of toxic compounds may make it difficult to dispose of a device which generates nitrogen gas from azides.
U.S. Pat. No. 6,299,743 to Oloman et al. (2001) discloses the electrochemical generation of nitrogen gas from organic nitrogen compounds, such as hydrazides (RCONHNH2), the corresponding organic hydrazino-carboxylates (RCO2NHNH2) and amino-guanidine salts (e.g. aminoguanide bicarbonate H2NNHC(NH)NH2.H2CO3). For example, the electro-oxidation of methyl hydrazinocarboxylate generates nitrogen gas with a stoichiometric efficiency of 0.5 moles per Faraday according to the putative reaction 4:CH3CO2NHNH2—>CH3CO2H+N2+2H++2e−  reaction4
Based on reaction 4 an electrical source with a current of at least 0.75 mA would be required to generate 0.33 ml STP/hour of nitrogen and a mass of 4 gram of methyl hydrazino-carboxylate would be needed to produce 1 litre STP of the gas.
Compounds having a high nitrogen content such as triazoles and tetrazoles have been investigated as non-azide nitrogen gas generant components in pyrotechnic compositions that may be useful as propellants or for inflating aircraft or automobile safety crash bags. Clearly, the explosive release of gases is not desirable in controlled electrolytic gas generators.